Polyester has excellent mechanical properties, thermal properties, chemical resistance, electrical properties, and formability and has been used in various applications.
However, because polyester decreases its mechanical properties due to hydrolysis when used over a long period of time or used in humid situations, various studies to suppress hydrolysis have been carried out. Particularly in films for a solar battery, the outdoor lifetime of 20 years or more is required and, therefore high hydrolysis resistance is required.
JP 2001-114881 A describes a process for producing polyester containing phosphate of an alkali metal or an alkaline earth metal. Further, JP 2007-277548 A describes a process for producing polyester containing inorganic phosphate, and phosphoric acid is used in combination in Examples. JP 2008-007750 A describes polyethylene terephthalate containing a buffer phosphorus compound, and a phosphorus compound is used in combination in Examples.
Only by a metal phosphate, as in the process of producing polyester disclosed in JP '881, initial COOH terminal groups can be suppressed. However, it is difficult to suppress an increase in the amount of COOH terminal groups due to hydrolysis, and sufficient hydrolysis resistance cannot be obtained in applications that require long-term durability such as solar battery application.
In the case of the process of producing polyester disclosed in JP '548, because the ratio of phosphoric acid to inorganic phosphate and their amount were inappropriate, inorganic phosphate readily turned into a foreign body and, although the short-term hydrolysis resistance was excellent, the hydrolysis resistance over a long period of time required in solar battery applications and the like was insufficient, and mechanical properties of a film was reduced by the foreign bodies.
In the case of polyethylene terephthalate disclosed in JP '750, appropriateness of the type, ratio, amount and the like of the phosphorus compound is insufficient and, therefore, hydrolysis resistance and mechanical properties are insufficient for solar battery applications.
Examples of the process of producing polyester include the method in which polycondensation is carried out after esterification reaction using dicarboxylic acid as a main raw material (direct polymerization method) and the method in which polycondensation is carried out after transesterification reaction using dicarboxylic acid ester as a main raw material (DMT method). The DMT method disperses particle components well and is excellent in suppression of foreign bodies, but has a problem in that the raw material cost is high compared to the direct polymerization method. On the other hand, the direct polymerization method allows an esterification reaction without a catalyst and further is very cost-effective because the raw material is inexpensive, but has a problem in that the hydrolysis resistance decreases because the amount of COOH terminal groups in the resulting polyester is high compared to the case of the DMT method.
Thus, there is a need to provide a process of producing a polyester composition suitable for use in a film with excellent hydrolysis resistance.